System for monitoring the quality of a communications channel with mirror receivers

ABSTRACT

A system is presented that monitors the quality of a communications channel with mirror receivers. A first receiver and a second receiver, coupled in parallel with the first receiver, receive a data signal transmitted over the communications channel. The second receiver generates an output signal. A signal integrity (SI) processor manipulates the output signal in order to determine the quality of the communications channel. The SI processor samples a phase-shifted version of the output signal, which has a phase shifted relative to a zero reference phase, and analyzes the phase-shifted version of the output signal for bit errors. In an embodiment, the SI processor manipulates the output signal to extract an eye diagram indicative of the quality of the communications channel. The SI processor non-intrusively determines the quality of the communications channel using the second receiver.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/767,748, filed Jan. 30, 2004, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to systems for monitoring the qualityof a communications channel.

2. Related Art

Communications systems can have backend data storage mechanisms ofvarious configurations. For example, in a direct-attached storageconfiguration, a server is directly connected to a data storage device.The server is connected with one or more clients on a Local Area Network(LAN) and controls client access to the data storage device. In anetwork-attached storage configuration, one or more servers areconnected with one or more clients on a LAN. The servers interface witha control device, which regulates access to one or more data storagedevices. The control device typically performs intelligent processing inorder to connect data storage devices onto various server paths.

The performance of communications networks typically degrades over timebetween system calibrations. A problem with control devices of typicalnetwork-attached data storage configurations is they provide minimal, ifany, communications channel quality monitoring. What is needed,therefore, is a system for monitoring the quality of a communicationschannel, particularly for communications systems having anetwork-attached data storage configuration.

SUMMARY OF THE INVENTION

The present invention is directed to a system for monitoring, withminimal additional hardware, the quality of a communications channel,for example, in a communications system having a network-attached datastorage configuration. In an embodiment of the present invention, thesystem includes mirror receivers. A first receiver receives a datasignal transmitted over the communications channel. A second receiver,coupled in parallel with the first receiver, receives the data signaland generates an output signal.

A signal integrity (SI) processor manipulates the output signal of thesecond receiver in order to estimate the quality of the communicationschannel. The SI processor samples a phase-shifted version of the outputsignal, which has a phase shift relative to a zero reference phase, andanalyzes the phase-shifted version of the output signal for bit errors.The SI processor non-intrusively estimates the quality of thecommunications channel using the second receiver.

In an embodiment of the present invention, the system for monitoring thequality of a communications channel further includes a phase acquisitionmodule coupled in communication with the SI processor. When triggered bythe SI processor, the phase acquisition module locks onto a phase of theoutput signal in order to establish the zero reference phase. A phaseshifting module is coupled in communication with the phase acquisitionmodule and the SI processor. When triggered by the SI processor, thephase shifting module generates a phase-shifted version of the outputsignal having a phase shifted relative to the zero reference phase.

In an embodiment, additional features of the present invention include aLink Integrity (LI) processor coupled in communication with the SIprocessor. The LI processor detects link-level errors in the outputsignal and aids the SI processor in estimating the quality of thecommunications channel. A bit error testing module compares bits of thephase-shifted version of the output signal to a pattern signal in orderto detect bit errors. A module coupled in communication with the SIprocessor enables a system operator to visualize the relative quality ofthe communications channel by generating an eye diagram extracted by theSI processor from the output signal.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to accompanying drawings. It isnoted that the invention is not limited to the specific embodimentsdescribed herein. Such embodiments are presented for illustrativepurposes only. Additional embodiments will be apparent to personsskilled in the relevant arts based on the teachings contained herein.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The present invention will be described with reference to theaccompanying drawings. The drawing in which an element first appears istypically indicated by the leftmost digit(s) in the correspondingreference number.

FIG. 1 illustrates an example environment in which the present inventioncan be used.

FIG. 2A illustrates a comparison of an ideal eye diagram to a degradedeye diagram.

FIG. 2B illustrates an eye diagram having multiple phase sampling pointsto the left and right of the center of the eye.

FIG. 2C illustrates an example structure of a Fibre Channel frame havinga fixed pattern that can be used for estimating a bit error rate forvarious phase-shift positions in an eye diagram of a received datasignal.

FIG. 3 illustrates a process flow chart of a method for monitoringcommunications channel quality, in accordance with an embodiment of thepresent invention.

FIG. 4 illustrates a process flowchart of phase camping and adaptivefine-tuning features of the method of FIG. 3, in accordance withembodiments of the present invention.

FIG. 5 illustrates a high-level diagram of a system for monitoringcommunications channel quality with mirror receivers, in accordance withan embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION Overview

The present invention is directed to a system for monitoring the qualityof a communications channel, for example, in a communications systemhaving a network-attached data storage configuration. In the detaileddescription that follows, the preferred embodiments of the presentinvention are presented in detail. While specific features,configurations, and devices are discussed in detail, this description isfor illustrative purposes, and persons skilled in the art will recognizethat other configurations and devices can be used to achieve thefeatures of the present invention without departing from the scope andspirit thereof.

Example Environment

Before describing the present invention, it is helpful to describe anexample environment in which the invention can be used. FIG. 1illustrates an example environment in which the present invention can beused. Communications systems typically have backend data storagemechanisms, which can have various configurations. For example,communications system 100 has a server 102 directly connected to a datastorage device 106 in direct-attached storage configuration. Server 102is connected with one or more clients 104 on a Local Area Network (LAN)and controls client 104 access to data storage device 106.

Communications system 100 also has a server 108, which in addition toserver 102, is connected to data storage devices 110 in anetwork-attached storage configuration. Servers 102 and 108 areconnected on LANs with one or more clients 104 and interface with acontrol device 112, which regulates access to data storage devices 110.Control device 112 typically performs intelligent processing in order toconnect data storage devices 110 onto various server paths. Inaccordance with an embodiment of the present invention, control device112 can include a system for monitoring the communications channelquality in order to alert a system operator to problems incommunications system 100.

Monitoring Communications Channel Quality using Eye Diagrams

FIG. 2A illustrates a comparison of an ideal eye diagram 200 to adegraded eye diagram 210. An eye diagram is formed by superimposingelectrical pulses corresponding to different transmitted data signalbits, and visually represents the quality of a communications channelthrough which the data signal is transmitted. For example, ideal eyediagram 200 of FIG. 2A has a wide eye opening 202, which is indicativeof a low noise, high quality communications channel. In ideal eyediagram 200, the signal level difference between different signal bitsis a maximum.

Degraded eye diagram 210 of FIG. 2A has a partially closed eye opening212, which is indicative of a noisy, low quality communications channel.In degraded eye diagram 210, the signal level difference betweendifferent signal bits is minimal as compared to ideal eye diagram 200. Adistorted eye diagram, such as degraded eye diagram 210 of FIG. 2A,alerts a system operator to problems in the communications system.Communications system problems such as chirped pulses, noise in thecircuit board or transmission medium, and aging of the communicationssystem components are manifested in an eye diagram and can cause the eyeopening to narrow.

Given an eye diagram indicative of the quality of a communicationschannel, a phase position corresponding to the maximum opening of theeye is the best phase, due to minimum possible noise tolerance, at whichto sample a signal transmitted through the channel. At the center of theeye opening, the signal level difference between two different signalbits is a maximum, but at the edges of the eye opening, the signal leveldifference between two different signal bits is minimal. Accordingly,the likelihood that a receiver will detect a signal bit in error is lowfor sampling phase positions near the center of the eye opening andincreases rapidly for sampling phase positions farther from the centerof the eye.

In accordance with the present invention, an eye opening for acommunications channel can be characterized by estimating a bit errorrate (BER) for various phase sampling positions in the eye diagram of areceived signal. FIG. 2B illustrates an example eye diagram 220 having anumber of phase sampling positions 224 to the left of a center 222 ofeye 220 and a number of phase sampling positions 226 to the right ofcenter 222. The invention is not, however, limited to these examplephase sampling positions or number of phase positions. Based on thedescription herein, one skilled in the relevant art(s) will understandthat the invention can be implemented with other phase samplingpositions and numbers of phase positions. In order to characterize thequality of a communications channel, the number of bit errors in thereceived data signal at each of the phase sampling positions and thetotal number of bits sampled can be accumulated, which in turn can beused to estimate a BER.

In order to perform bit-by-bit error detection at the receiver, a hostcontinuously transmits signal frames containing a pre-defined repeatedbit pattern. For example, FIG. 2C illustrates an example structure of aFibre Channel frame 230 having a fixed 40-bit repeated pattern 232 thatcan be used for estimating a BER for various phase-shift positions in aneye diagram of a received data signal. The invention is not, however,limited to the example of a Fibre Channel network protocol. Based on thedescription herein, one skilled in the relevant art(s) will understandthat the invention can be implemented with other network protocols.Bit-by-bit error detection at the signal level permits more preciseevaluation of the quality of a communications channel than word or frameerror detection at the link level.

Active Link Integrity/Signal Integrity Method of Monitoring ChannelQuality

FIG. 3 illustrates a process flow chart of a method for monitoringchannel quality 300, in accordance with an embodiment of the presentinvention, referred to herein as Active Link Integrity/Signal Integrity(“Active LI/SI”) analysis. In step 302, a data signal is received. In anembodiment of the present invention, Active Link Integrity (“Active LI”)analysis is automatically performed in step 306.

In step 306, the received data signal is analyzed for link-level errors.Step 306 shows four example link-level errors, which reflect anapproximate number of bit errors in the received data signal:8-bit/10-bit running disparity errors 301, character and word errors303, order set violations 305, and cyclic redundancy code errors 307.The invention is not, however, limited to the detection and accumulationof these example link-level errors. Based on the description herein, oneskilled in the relevant art(s) will understand that the invention can beimplemented to detect and accumulate other link-level errors.

In step 309, a bit error rate (BER) can be estimated according toaccumulated link-level errors. In an embodiment, step 309 is implementedin firmware, independent of method 300 steps shown in FIG. 3. Whileaccumulated 8-bit/10-bit running disparity errors 301 can be used toestimate a BER, they are preferably used as a warning mechanism. Also,BERs higher than approximately 10⁻⁶ are not reliably estimated byaccumulated CRC errors 307, and BERs higher than approximately 10⁻³ arenot reliably estimated by accumulated order set violations 305.

Accordingly, Active LI analysis provides a gross characterization of thequality of a communications channel. An advantage of Active LI analysisis a host is not required to continuously send data signal framescontaining a pre-defined repeated pattern; therefore, Active LI can beperformed on a received data signal under normal operating conditions.Additionally, Active LI analysis does not disturb the received datasignal in order to characterize the quality of a communications channel.

To obtain a more accurate characterization of the quality of acommunications channel, a system operator can request Active SignalIntegrity (“Active SI”) analysis in step 308. In an embodiment of thepresent invention, Active SI is not automatically performed becauseActive SI analysis manipulates the received data signal in order tocharacterize the quality of a communications channel.

In step 308, a zero reference phase is established by determining thecenter 222 of eye 220 of FIG. 2B. In step 310 of FIG. 3, the phase ofthe received data signal is shifted to a sampling phase, producing aphase-shifted data signal. For example, a sampling phase can be one ofthe thirty-two phase sampling positions 224 and 226 shifted to the leftor right of center 222 of eye 220 of FIG. 2B, respectively. Active SIanalysis disrupts the received data signal by shifting the received datasignal phase in order to sample at different phase positions in the eyediagram of the received data signal.

In step 312 of FIG. 3, Active SI analysis initiates sampling thephase-shifted data signal. As shown in FIG. 3, the phase-shifted datasignal can simultaneously be analyzed in step 306 according to Active LIanalysis. Active LI analysis aids Active SI in synchronization. In step314, Active SI analyzes the phase-shifted data signal bit-by-bit for biterrors. As described above in conjunction with FIG. 2C, Active SIrequires that a host send data signal frames containing a pre-definedpattern signal in order to detect phase-shifted data signal bits inerror. In step 316 of FIG. 3, a number of phase-shifted data signal bitsare detected and a number of phase-shifted data signal bits in error areaccumulated.

In step 318, the accumulated number of bits detected in error iscompared to an error threshold. If the accumulated number of bits inerror exceeds the error threshold, then sampling is discontinued in step320. Otherwise, in step 324, the accumulated number of bits detected iscompared to a sampling window duration, which can be expressed as anumber of bits. If the accumulated number of bits detected is less thanthe duration of the sampling window, then bit error detection resumes instep 314. When the accumulated number of bits detected equals or exceedsthe duration of the sampling window, sampling is discontinued in step320.

In step 309, a BER is estimated according to the accumulated number ofbits in error relative to the accumulated number of bits detected. In anembodiment, step 309 can be implemented in firmware, independent ofmethod 300 steps shown in FIG. 3, as described above. In step 322 an eyediagram is updated according to the BER estimated in step 310. Asdescribed above in conjunction with FIG. 2A, a system operator canvisually assess the quality the communications channel according to thedegree of eye opening. For example, a wide eye opening, such as eyeopening 202 of FIG. 2A, indicates a low noise, high qualitycommunications channel. A partially closed eye opening, such as eyeopening 212 of FIG. 2A, indicates a noisy, poor quality communicationschannel.

Adaptive Active LI/SI Method of Monitoring Channel Quality

Noise in a communications system, such as sinusoidal jitter, introduceslimitations on sampling window duration and the number of sampling phasepositions that can be implemented with Active LI/SI method 300 of FIG.3. The presence of high sinusoidal jitter in a communications systemrequires a short duration sampling window, otherwise the high jitter cancause the system to lose its lock on the received data signal. However,estimating higher BERs requires a long duration sampling window.

FIG. 4 illustrates a process flowchart of phase camping and adaptivefine-tuning features of Active LI/SI method 300 of FIG. 3, in accordancewith embodiments of the present invention. In FIG. 4, a long durationsampling window is achieved by sampling the phase-shifted data signalfor multiple short duration sampling windows. This process is referredto herein as phase camping.

In step 402, a phase of the received data signal is shifted to asampling phase position. In step 404, sampling is initiated for a shortduration sampling window. In step 406, the phase-shifted data signal isanalyzed for bit errors. In step 408 a number of bit errors and a numberof bits detected are accumulated. In step 410, the accumulated number ofbits detected is compared to the duration of a desired sampling window.If the accumulated number of bits detected exceeds the duration of thedesired sampling window, sampling is discontinued in step 418 and theeye diagram is updated in step 420. Otherwise, the zero reference phaseis reestablished in step 412 before sampling is resumed in step 404 foran additional short duration sampling window. In other words, in step412, the center of the eye diagram is re-determined. By reestablishingthe zero reference phase between the multiple short duration samplingperiods, the phase camping feature of method 400 increases thelikelihood that synchronization can be maintained in the presence ofhigh sinusoidal jitter or other noise in the communications channel. Inan embodiment of the present invention, a random delay is introducedbetween samples, after the zero reference phase is reestablished.

Step 412 also illustrates a feature of method 400 that substantiallyreduces the amount of time required to reestablish the zero referencephase, in accordance with an embodiment of the present invention. Thismethod is referred to herein as adaptive fine-tuning. For example, instep 414, a first shift from the sampling phase to the zero referencephase, which corresponds to center 222 of eye 220 of FIG. 2B, isaccomplished with 10-bit adaptive feedback control signal accuracy.First shift 414 is a gross estimation of center 222 of eye 220 of FIG.2B. In step 416, a second shift from the reference phase established instep 414 to the zero reference phase is accomplished with 16-bitaccuracy. A single shift from the sampling phase to the zero referencephase with 16-bit accuracy would require significantly more time thanthe combination of gross-estimation shift 414 and fine-estimation shift416. The invention is not, however, limited to these example values ofgross and fine tuning bit accuracy (e.g., depending on theimplementation, the gross tuning step could use ten to twelve bits andthe fine tuning step could use fourteen to sixteen bits). Based on thedescription herein, one skilled in the relevant art(s) will understandthat the invention can be implemented with other values of gross andfine tuning bit accuracy.

As described above, sinusoidal jitter in a communications system limitsboth sampling window duration and the number of sampling phase positionsthat can be implemented with Active LI/SI method 300 of FIG. 3. Examplevalues of maximum sampling window duration and number of sampling phasepositions for a high sinusoidal jitter communications system are 500bits and seven phase sampling positions (three to the left of center 222of eye 220 of FIG. 2B, center 222, and three to the right of center222), respectively. Example values of maximum sampling window durationand number of sampling phase positions for a low sinusoidal jittercommunications system are 32000 bits and seventeen sampling phasepositions (eight to the left of center 222 of eye 220 of FIG. 2B, center222, and eight to the right of center 222), respectively. The inventionis not, however, limited to these example sampling window durations andnumbers of phase sampling positions. Based on the description herein,one skilled in the relevant art(s) will understand that the inventioncan be implemented with other sampling window durations and numbers ofphase sampling positions.

In an embodiment of the present invention, algorithms that determineActive LI/SI operating parameters can be implemented in firmware,independent of method 300 steps shown in FIG. 3. Such Active LI/SIoperating parameters include: a quantity of phase sampling positions toanalyze, which phase sampling positions to analyze, which pre-defineddata pattern to transmit, the maximum duration of a sampling window, anda bit error threshold. In another embodiment, a system operator canselect the Active LI/SI operating parameters individually or selectprograms of pre-defined operating parameters according to assessedchannel conditions.

A System for Monitoring Channel Quality with Mirror Receivers

FIG. 5 illustrates a high-level diagram of a system for monitoring thequality of a communications channel with mirror receivers, in accordancewith an embodiment of the present invention. A communications system 500having a network-attached storage configuration is shown in FIG. 5.

In FIG. 5, communications system 500 includes a server 502 coupled to acontrol device 520, which is coupled to a bank of data storage devices518. Control device 520 intelligently connects data storage devices 518onto various server paths. While communications system 500 is depictedas a uni-directional communications system, the invention is not,however, limited to this example configuration. Based on the descriptionherein, one skilled in the relevant art(s) will understand that theinvention can also have a bi-directional configuration.

In FIG. 5, control device 520 includes a digital circuit 504 and ananalog circuit 512. According to an embodiment of the present invention,digital circuit 504 contains mirror receivers, a first receiver 506 anda second receiver 508, coupled to receive an original data signal 501transmitted by server 502. Second receiver 508 generates an outputsignal 503. A signal integrity (SI) processor 510 is coupled to receiveoutput signal 503 and manipulates output signal 503 in order to monitorthe quality of the communications channel.

SI processor 510 is coupled in communication with analog circuit 512.Analog circuit 512 contains a phase acquisition module 514 coupled incommunication with SI processor 510. When triggered by SI processor 510,phase acquisition module 514 locks onto output signal 503 to establish azero reference phase (analogous to center 222 of eye 220 of FIG. 2B). Aclock recovery module 518 is coupled in communication with phaseacquisition module 514 to aid in synchronizing bits of output signal 503with an embedded clock signal. A phase shifting module 516 is alsocoupled in communication with phase acquisition module 514 and SIprocessor 510. When triggered by SI processor 510, phase shifting module516 shifts a phase of output signal 503 to a sampling phase position(analogous to a phase position to the left or right of center 222 of eye220 of FIG. 2B) shifted relative to the zero reference phase.

SI processor 510 samples the phase-shifted version of the output signalgenerated by phase shifting module 516, and analyzes the phase-shiftedversion of the output signal bit-by-bit for bit errors. In anembodiment, SI processor 510 further includes a bit error testing module520 that compares bits of the phase-shifted version of the output signalto a pattern signal in order to detect bit errors. SI processor 510 canfurther include error accumulation registers 522 for storing a number ofbits detected and a number of bit errors so that SI processor 510 canestimate a communications channel quality measurement according to theaccumulated number of bit errors relative to the accumulated number ofbits detected. The communications channel quality measurement can be anestimated BER.

In an embodiment of the present invention, communications system 500further includes a module 524, coupled in communication with SIprocessor 510, for enabling a system operator to visualize the qualityof the communications channel. Module 524 can generate an eye diagram,which is indicative of the quality of the communications channel,extracted from output signal 503 by SI processor 510. Communicationssystem 500 can also include a Link Integrity (LI) processor 526 coupledin communication with SI processor 510. LI processor 526 detectslink-level errors in output signal 503 and aids SI processor 510 inestimating the quality of the communications channel.

When SI processor 510 discontinues sampling, it triggers phaseacquisition module 514 to reestablish the zero reference phase. SIprocessor 510 then triggers phase shifting module 516 to either shiftthe phase of output signal 503 back to the previous sampling phaseposition or to a new sampling phase position. The mirror receiverconfiguration of communications system 500 is advantageous because SIprocessor 510 manipulates output signal 503 in order to extract acommunications channel quality measurement, and thus does not disturbthe integrity of original data signal 501.

CONCLUSION

The present invention has been described above with the aid offunctional building blocks illustrating the performance of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed. Any such alternate boundaries are thus within the scope andspirit of the claimed invention. One skilled in the art will recognizethat these functional building blocks can be implemented by discretecomponents, application specific ICs, processors executing appropriatesoftware and the like or any combination thereof.

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. Thus, the breadth and scope of thepresent invention should not be limited by any of the above-describedexemplary embodiments, but should be defined only in accordance with thefollowing claims and their equivalents.

1. A method for monitoring the quality of a communications channel,comprising: receiving a data signal transmitted over the communicationschannel; processing said data signal to produce an output signal;phase-shifting said output signal to produce a phase-shifted version ofsaid output signal having a phase shifted relative to a reference phase;and detecting bit errors by sampling said phase-shifted version of saidoutput signal, thereby monitoring the quality of the communicationschannel.
 2. The method of claim 1, wherein said step of phase-shiftingcomprises synchronizing data bits of said output signal with a clocksignal embedded in said output signal.
 3. The method of claim 1, whereinsaid step of detecting includes accumulating a count of detected bitsand a count of said bit errors, and estimating the communicationschannel quality according to said count of said bit errors relative tosaid count of detected bits.
 4. The method of claim 1, wherein said stepof detecting includes comparing bits of said phase-shifted version ofsaid output signal to bits of a pattern signal.
 5. The method of claim1, wherein said step of detecting includes detecting link-level errorsin said phase-shifted version of said output signal.
 6. The method ofclaim 1, further comprising the step of enabling a system operator tovisualize the quality of the communications channel.
 7. The method ofclaim 6, further comprising generating an eye diagram from saidphase-shifted version of said output signal to characterize the qualityof the communications channel.
 8. A method for monitoring the quality ofa communications channel, comprising: receiving a data signaltransmitted over the communications channel; generating first and secondoutput signals based on said received data signal; manipulating saidsecond output signal to monitor the quality of the communicationschannel without disrupting said first output signal.
 9. The methodaccording to claim 8, wherein said step of manipulating includes phaseshifting said second output signal to generate a phase-shifted versionof said output signal.
 10. The method of claim 8, wherein said step ofmanipulating includes the steps of: detecting bit errors by samplingsaid phase-shifted version of said second output signal; accumulating acount of detected bits and a count of said bit errors; and estimatingthe communications channel quality according to said count of said biterrors relative to said count of detected bits.
 11. The method of claim10, further comprising the step generating an eye diagram from saidsecond output signal to characterize the quality of the communicationschannel.
 12. The method of claim 10, further comprising the step ofdetecting link-level errors in said second output signal.